Second set of eggs to hatch early

I hatched a batch of eggs last month, hatching began on day 20, only 12 out of 35 hatched, 14 eggs were fully developed, but never hatched, one chick was deformed, one was a double yolk with twins that died about 13-15 days (didn't know it was a double yolk until I opened it). It was just horrible!
So I put another batch in, and, as recommended by some of you, I tried dry hatching. Well, it's day 20, and I woke up this morning to 3 chicks. The temperature is set at 99.5. What's going on?

What type of chicken are they? Many breeds, especially bantams will occaisonally hatch early. The smaller bantams may hatch on day 19-20, and I've even heard of Seramas hatching on day 17 or 18. As long as the chicks seem healthy, the early hatching time is probably normal. Late hatches are often more worrisome and can be a sign of weak chicks.

I think that it is also possible that the therometer is off a degree or two. Higher temperatures can cause early hatches. Do you have another therometer or two that you can put in the incubator and see if they read the same temperature? I know that I have some therometers that consistently read higher than the actual temperature. As a result, my hatches used to go late due to cooler temperatures (they would read 100, but be less than 99 degrees F.), until I discovered that the therometer was wrong.

I have Rhode Island reds, so that's probably not why. Last time I had 3-4 hatch with yolks not absorbered, and that's what worries me. I have a styrofoam bator and can't really see in there, as the look through parts are scratched, so I'll have to test with another thermometer after hatch, so I don't have to open it